1. Technical Field
This invention relates generally to the field of flexographic printing equipment and particularly to methods and apparatus for mounting flexographic plates to the plate cylinders.
2. Background Art
The flexographic printing process utilizes an elastomeric printing plate which generally has raised or letterpress printing features. The plates may be made of various materials, including natural and synthetic rubbers, plastics and photopolymer plastic. The finished flexographic plate is mounted to the surface of a plate cylinder in various ways, and printing is generally accomplished in the letterpress manner by passing the paper or other substrate to be printed through the nip between the plate cylinder and a backup or impression cylinder.
In many modern printing applications, particularly where multiple printing images are formed on a single plate, and in multicolor printing, the precision with which the plate is mounted to the cylinder is critical to successful printing. Proper alignment of multiple images formed on a single plate will allow each image to be printed at its proper location on the substrate, a particularly significant factor when printing is performed upon relatively small substrates such as tags and labels. Highly accurate mounting of the plates is also essential in multi-color printing wherein several plates on separate plate cylinders are used to lay down the various colors on the substrate in sequence. The image impressed by each plate in its turn upon the substrate must be precisely registered with the previously printed images to avoid overlap of the images or mixing of the inks.
To aid in the proper mounting of flexographic plates, scribe lines or register marks are generally formed on the face of the plate outside of the image area which are parallel to the "vertical" and "horizontal" axes of the image or images on the plate. In simple plate mounting procedures, adequate for one color jobs, a pressman can mount the plates by lining up the scribe marks with horizontal and circumferential lines scratched on the surface of the plate cylinder. Final adjustment of the printing images on two separate plate cylinders in two color printing can then sometimes be accomplished with the cylinders mounted on the press by moving the two cylinders sideways and circumferentially with respect to one another until the printed images on sample runs are in registry. However, such simple mounting procedures are totally inadequate for three and four color printing or for printing with plates bearing multiple designs. Errors and inaccuracies in the mounting of the plates is costly to correct because of the machine down-time and the attention required by highly skilled pressmen to obtain the desired alignment of the images.
To minimize the amount of on-press time required to accurately align images printed by plates which had been inaccurately mounted, various machines have been developed and are being commercially used to allow more accurate mounting of the plate to the cylinder. However, considerable skill is required on the part of the machine operator to properly mount plates utilizing such machines, and the process of mounting is often time consuming and subject to human error.